cosα=-4/5,α是第三象限角
sinα=-√(1-cos^2α)=-3/5
sin(3π/4-α)=sin3π/4cosα-cos3π/4sinα=-7√2/10
cos(π/4+2α)=cosπ/4cos2α-sinπ/4sin2α=√2/2(cos^2α-sin^2α-2sinαcosα)=-7√2//50